Valgrind: Add a fetch_layout command to waterfall.sh.
Valgrind: Add a fetch_layout command to waterfall.sh.

Instead of fetching 3 build results from all bots, fetch 30 from the
layout tests bot only.

Review URL: https://codereview.chromium.org/1400273002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#353877}

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